libvirt_xml: Add memoryBacking tag for using hugepage in vmxml.

The tag tells the hypervisor that the guest using hugepages,
instead of the normal native page size.

Signed-off-by: Wei,Jiangang <weijg.fnst@cn.fujitsu.com>
virttest.staging.utils_memory: Add functions to get/set hugepage parameters

These functions can be used to obtain the parameters of hugepage, such as
used/free/reserved memory,and enable/disable the transparent hugepage feature.